This invention relates to a silicon nitride sintered body and more particularly, to a silicon nitride sintered body that shows a small reduction of strength at room temperature and at high temperatures and that shows superior oxidation resistance. Furthermore, the invention relates to a method for making the same.
Hitherto, various heat resistant metallic alloys have been used as materials for gas turbines, heat exchangers, etc. However, recently, the use of various ceramics in place of these alloys has been attempted under severer conditions, which these alloys cannot stand, especially at high temperatures of 1000.degree. C. or higher for purposes of improvement of thermal efficiency and energy saving. Among those ceramics, a silicon nitride (Si.sub.3 N.sub.4) sintered body has been noticed as a structural material along with a silicon carbide (SiC) sintered body owing to its superior strength, wear resistance and oxidation resistance, small coefficient value of thermal expansion, excellent strength at high temperatures and chemical stability.
Development of ceramic gas turbines has been made, according to which improvement of thermal efficiency of gas turbines is attained by using ceramics of excellent heat resistance in place of the conventional heat resistant alloys. Characteristics required for ceramic materials used for gas turbines are as follows:
(1) They must have high strength at high temperatures so that they can stand strong centrifugal force due to high-speed rotation at high temperatures.
(2) They must withstand thermal shock at combustion and at stopping.
(3) They must withstand strong oxidation conditions at high temperatures.
All of these conditions must be satisfied for practical use.
As materials of the gas turbines, SiC and Si.sub.3 N.sub.4 materials have been studied, but SiC has problems in the above (1) and (2) and Si.sub.3 N.sub.4 has problems in (1) and (3).
Silicon nitride itself is poor in sinterability because of its covalent bonding and hence it is difficult to obtain a sintered body with high density and high strength from silicon nitride along. Therefore, it has been attempted to add Al.sub.2 O.sub.3, MgO and oxides of rare earth elements during sintering of silicon nitride as a sintering aids. The added sintering aid reacts with SiO.sub.2 present on the surface of silicon nitride powder in the sintering process to form a glass phase of low melting point and accelerate sintering, whereby the final sintered body is composed of silicon nitride and a grain boundary phase.
Therefore, in order to obtain sintered bodies that exhibit high strength at high temperature and superior oxidation resistance, the sintering aids used must have a sintering acceleration effect, and also must form a grain boundary phase with high strength at high temperature and further form an oxide film good in oxidation resistance.
Many proposals have been made for silicon nitride sintered bodies excellent in mechanical strength and oxidatin resistance.
Japanese Patent Kokai (Laid-Open) No. 197468/86 discloses a silicon nitride sintered body comprising at least 80 wt% (of the total) of .beta.-silicon nitride and the balance being a solid solution of zirconia and ytterbium oxide as a grain boundary phase. This grain boundary phase is composed of a solid solution of 1-10 mol% of ytterbium dissolved in zirconia. According to this patent publication, when the amount of ytterbium oxide is outside the above range, oxidation resistance at high temperatures is deteriorated, sinterability is poor and densification is difficult.
Japanese Patent Kokai (Laid-Open) No. 116671/80 discloses a method of improving high-temperature strength which comprises adding Al.sub.2 O.sub.3 or AlN and rare earth oxide to silicon nitride to form a grain boundary phase composed of crystal of Si.sub.3 N.sub.4.R.sub.2 O.sub.3 [R represents an element of group IIIa (scandium series) of the periodic table] type structure to make a material with high strength at high temperature. According to this method, for example, when y (yttrium) is employed as the element of Group IIIa, Si.sub.3 N.sub.4.Y.sub.2 O.sub.3 is precipitated as a grain boundary phase to provide a high strength at three-point bending of about 110 Kg/mm.sup.2 at room temperature and 1200.degree. C., but there is a problem in oxidation resistance. This method is characterized by precipitation of Si.sub.3 N.sub.4.R.sub.2 O.sub.3 (R is an element of Group IIIa of the periodic table) as grain boundary phase to improve high-temperature strength, but evaluation of oxidation resistance has revealed that a great increase in weight of about 2.0 mg/cm.sup.2 is caused in oxidation test at 1300.degree. C. for 300 hours. Thus, there is a problem in use of this material at high temperatures where characteristics of silicon nitride can be actually exhibited.